Japanese Patent Publication No. 3363761 discloses a scan converter for converting progressive scan video data into interlaced scan video data. The scan converter comprises a frame memory and line memories, and one line of progressive scan video data sent from an image pickup means is written into the line memory at normal transfer rate in a certain period. In the next period, the video data written into the line memory is read out therefrom and written into the frame memory at twice the normal transfer rate in the first half of the period, while another line of the progressive scanning video data is written into the above line memory at the normal transfer rate, and, in the latter half of the period, the video data stored in the frame memory is read out therefrom at twice the normal transfer rate. Thus, the scan converter can convert the progressive scanning video data into the interlaced scanning video data using a single port type frame memory, so it can reduce the cost.
However, because it is necessary for the scan converter to equalize the period in which one line of the video data is written into the frame memory and the period in which one line of the video data is read out from the frame memory, it is necessary for the scan converter to synchronize the progressive scan video data and the interlaced scan video data. For example, when a signal extracted from the progressive scan video data is used as the horizontal synchronization signal, the interlaced scan video data has to synchronize with the progressive scan video data. Conversely, when a signal extracted from the interlaced scan video data is used as the horizontal synchronization signal, the image pickup means has to have a function which synchronize the video data by an external signal.
Furthermore, because this scan converter synchronizes the progressive scan video data and the interlaced scan video data on a line-by-line basis, the progressive scan video data and the interlaced scan video data also synchronize with each other on a frame-by-frame basis. Therefore, only an image pickup means having the same frame rate as the interlaced scan video data can be used, and the image pickup means having a different frame rate can not be used.
Still furthermore, because the scan converter needs to use line memories, the cost may increase.